1. Field of the Invention
The present invention relates to production of copper powder and, more particularly, to a method for producing finely divided spherical copper powder for use in conductive pastes.
2. Description of the Prior Art
To the present, conductive pastes have been used for formation of thick film conductors such as electrodes or conductive patterns in multilayered-type electronic parts or multilayered printed circuit boards. For example, internal electrodes of multilayered or monolithic ceramic capacitors are formed by applying a conductive paste on ceramic sheets by screen printing, drying the printed ceramic sheets, and baking them in a non-oxidizing atmosphere such as nitrogen atmosphere or in a weak oxidizing atmosphere.
The recently employed conductive paste includes copper powder dispersed in an organic vehicle along with glass frit. It is required to use copper powder which is free from oxidation and has uniform particle size. If coarse particles are present in the copper powder, they would fill meshes of the printing screen. Further, a wide particle size distribution makes it difficult to form thick film conductors with a uniform thickness. If copper powder oxidizes, it leads to formation of thick film conductor with bad appearance and poor solderability. In addition, since copper powder is gradually oxidized in air, special cares is required for storage of the copper powder.
To solve such problems, it has been proposed to protect copper powder with an antioxidant, for example, by incorporating an antioxidant into a conductive paste containing finely divided copper powder, or by a wet treating process including the steps of immersing finely divided copper powder in a solution of boric acid or stearic acid dissolved in an alcoholic solvent (generally, methyl alcohol) and then heating the wet copper powder after filtration to form a protective coating of boric acid or stearic acid on the copper particles.
However, the above anti-oxidizing treatments of the prior art is time-consuming. In the former process, the antioxidant must be incorporated into a conductive paste prepared by dispersing a previously ground copper powder in an organic vehicle along with glass frit. In the latter wet process which has been put to practice use, it is required to perform the grinding procedure and anti-oxidizing treatment separately. In addition, the wet process leads to the formation of volatile esters, causing the evaporation of boric acid or stearic acid during stirring, mixing and heating steps with the copper powder.